MC68030 Datasheet, PDF (305/602 Pages) Motorola, Inc – ENHANCED 32-BIT MICROPROCESSOR

Memory Management Unit
PHYSICAL ADDRESS
This 24-bit field contains the physical address bits (A31-A8) from the page descriptor cor-
responding to the logical address. When the page size is larger than 256 bytes, not all bits
in the physical address field are used. All page index bits of the logical address are trans-
ferred to the bus controller without translation.
9.5 TRANSLATION TABLE DETAILS
The details of translation tables and their use include detailed descriptions of the
descriptors, table searching, translation table structure variations, and the protection
techniques available with the MC68030 MMU.
9.5.1 Descriptor Details
The descriptor details include detailed descriptions of the short- and long-format descriptors
used in the translation trees. The fields that apply to all descriptors are described in the first
paragraph.
9.5.1.1 DESCRIPTOR FIELD DEFINITIONS. All descriptor fields are used in more than
one type of descriptor. This section lists these fields and describes the use of each field.
DT
This 2-bit field contains the descriptor type; the first two types apply to the descriptor it-
self. The other two types apply to the descriptors in the table at the next level of the tree.
The values are defined as follows:
$0 INVALID
This code identifies the current descriptor as an invalid descriptor. A table search
ends when an invalid descriptor is encountered.
$1 PAGE DESCRIPTOR
This code identifies the current descriptor as a page descriptor. The page descrip-
tor is a normal page descriptor when it resides in a page table (in the bottom level
of the translation tree). A page descriptor at a higher level is an early termination
page descriptor. A table search ends when a page descriptor of either type is en-
countered.
MOTOROLA
MC68030 USER’S MANUAL
9-17